The present invention is generally related to oxygen sensors, and, more particularly, systems and methods for measuring the concentration of oxygen in a flow of gas.
With continuous improvements and stringency in environmental regulations and advances in emission control technology, there is an intense demand for low-cost, high-sensitivity gas sensors for better control of combustion in order to minimize pollutant emission while improving energy efficiency. One of the most important gas sensors is the solid-state oxygen sensor for control of the air-to-fuel ratio in automobiles, furnaces, and other combustion processes. While potentiometric oxygen sensors have been widely used for control of stoichiometric combustion, they are not adequately sensitive to changes in oxygen concentration when the partial pressure of oxygen in a sample gas is too close to that of a reference gas, typically air, because of the logarithmic response. On the other hand, an amperometric, or a limiting-current type, oxygen sensor exhibits a linear dependence on oxygen concentration in the sample gas. Amperometric sensors are, therefore, more suitable for control of clean-burn combustion.
For a traditional amperometric oxygen sensor, a porous ceramic layer, or a cap with a laser-drilled hole, is used as a diffusion barrier to control the inflow of oxygen. The characteristics of such a sensor depend critically on the micro structure of the diffusion barrier, or the size of the hole. The disadvantages associated with this design include: (i) the pore or hole dimension is difficult to control; and (ii) the pores or hole can be readily blocked by particulates in the sample gas to be monitored.
To overcome these difficulties, mixed-conducting ceramic membranes have been used as the diffusion barrier for amperometric sensors, as described, for instance, in U.S. Pat. No. 5,543,025 to Garzon, et al. To date, however, the mixed conductors typically have been formed of lanthanum manganese oxide (LSM), lanthanum strontium cobalt oxide (LSC), and terbia-yttria stabilized zirconia (Tb-YSZ). The stability of these mixed conductors is questionable and the reliability of a solid-state gas sensor depends mainly on the stability of the sensing components, particularly the one in contact with exhaust. For example, the stability and reliability of a sensor based on a mixed-conducting membrane depend critically on the stability of the dense mixed-conductor membrane exposed to exhaust containing various pollutants at temperatures up to 1100xc2x0 C.
It is well known that LSM, LSC, and Tb-YSZ are not very stable in gases containing unburned hydrocarbons and sulfur-containing compounds at high temperatures. These mixed conductors may undergo irreversible structural changes when exposed to unburned hydrocarbons. Further, they may react with sulfur-containing gases at temperatures up to 1100xc2x0 C., forming reaction products at the surfaces that may alter the electrical properties of the materials. Accordingly, the performance of a sensor based on these mixed conductors may change during the course of operation, leading to drift in sensor output (or lack of stability), and even to sensor failure.
In addition to the chemical stability, the transport properties of the mixed conductors used as the diffusion barrier must not change significantly over the oxygen partial pressure range of interest in order to achieve wide-range oxygen detection. Therefore, the mixed conductors used as the diffusion barrier must have excellent stability under operating conditions to achieve stability, reliability, and reproducibility.
Therefore, there is a need for improved oxygen sensors, systems and methods which address these and other shortcomings of the prior art.
The present invention is generally directed to an emission control system for determining a concentration of oxygen in a flow of gas using a sensor. In a preferred embodiment, the system includes a sensor which incorporates a diffusion barrier, an electrolyte material, and a counter-electrode. Preferably, the counter-electrode is configured to support the diffusion barrier, and the electrolyte material is disposed between the diffusion barrier and the counter-electrode.
In accordance with another aspect of the present invention, the present invention can also be viewed as providing a method for determining the concentration of oxygen in a flow of gas. In this regard, the method can be broadly summarized by the following steps: providing a sensor in the flow of gas, and providing, from the sensor, a signal corresponding to the concentration of oxygen in the flow of gas. The sensor preferably includes a diffusion barrier, an electrolyte material, and a counter-electrode, with the counter-electrode being configured to support the diffusion barrier, and the electrolyte material being disposed between the diffusion barrier and the counter-electrode.
In accordance with another aspect of the present invention, an alternative method for determining the concentration of oxygen in a flow of gas can be summarized by the following steps: providing a counter-electrode; providing an electrolyte material; depositing the electrolyte material on the counter-electrode; providing a diffusion barrier; depositing the diffusion barrier on the electrolyte material; and co-firing the electrolyte material, the counter-electrode, and the diffusion barrier.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.